1. Field of the Invention
The present invention generally relates to color processing, and more particularly to a color processing system and apparatus adaptable to illumination.
2. Description of Related Art
An image sensor, such as a complementary metal-oxide-semiconductor (CMOS) image sensor, is a device that converts an optical image into electronic signals. The image sensor has been widely used in a variety of applications such as cell phones and cameras. The image sensor, in general, can convert visual light but also infrared light, which however causes discoloring phenomenon. As a result, an infrared band-stop filter is usually applied before the image sensor to block the infrared light.
Sensitivity is an important parameter that characterizes efficiency of the image sensor. The sensitivity of the CMOS image sensor, for example, is commonly limited to area of photo sensors that compose the image sensor. As the density of the image sensor increases, sensitivity thus decreases.
In a low-light condition, the unwanted infrared light mentioned above may become desirable, for example, in a surveillance system, to increase visibility. U.S. Pat. No. 7,239,344, entitled “Camera and device for switching optical filters,” the disclosure of which is hereby incorporated by reference, discloses a scheme in which two optical filters are switched by a motor according to illumination. In day light, a normal IR-cut filter is used to block the infrared light to avoid discoloring; in a night mode, another optical filter is applied to pass the infrared light. This scheme, however, consumes substantial power and reduces lifetime of an imaging device such as a camera.
U.S. Pat. No. 8,408,821 entitled “Visible and infrared dual mode imaging system,” the disclosure of which is hereby incorporated by reference, discloses a dual-band filter that passes visible light band and an infrared light band centered at 950 nm. However, an infrared light source is required, in a night mode, to emit infrared light that may be captured by the infrared light band of the dual-band filter. In day light, there is still infrared light passed by the infrared light band, which causes discoloring.
U.S. Pat. No. 7,864,233 entitled “Image photographing device and method,” the disclosure of which is hereby incorporated by reference, discloses a color filter array (CFA) that contains, among others, a filter passing visible plus infrared light band and another filter passing visual light band. Infrared band may then be calculated by the difference between the sensed signals of the two filters mentioned above, and may then be used to avoid the discoloring. In this scheme, more processing is required to filter out infrared signals, and sampling rate is substantially reduced.
U.S. Pat. No. 8,619,143 entitled “Image sensor including color and infrared pixels,” the disclosure of which is hereby incorporated by reference, discloses a scheme that uses a notch type IR-cut filter that blocks a transition band between the visible light band and the infrared band. This scheme, however, suffers low sensitivity in low-light condition.
U.S. Pat. No. 8,508,633 entitled “Image device with color and brightness signal processing,” the disclosure of which is hereby incorporated by reference, discloses a scheme that uses complementary color (e.g., W−R) of image pipelines as a color filter array (CFA). As color filter spectral response depends on material used, the response therefore may not be easily acquired or optimized. Moreover, in this scheme, white pixel value is directly used as a brightness signal, therefore causing color reproduction error, especially in color saturation.
For the foregoing reasons, a need has thus arisen to propose a novel color processing system for overcoming disadvantages of the conventional color processing systems.